In the way that one harbours prejudices against MPs one barely knows, I have long marked down Alan Duncan, the shadow leader of the House, in my mind.

There are half-remembered mini scandals in his past that do not redound to his credit. He also seems to have been very adept at attaching himself to whoever is the Tory leader of the day, and has therefore veered opportunistically between being an unreconstructed Thatcherite and trendy moderniser.

So when I read that he had been secretly filmed by a campaigner called Heydon Prowse complaining about MPs’ reduced standard of living, my immediate and unthinking response was one of delight. Mr Duncan appears so creepily and implausibly virtuous in public on the subject of MPs’ expenses, even though some of his own claims were dodgy. Here he was speaking his own mind, and unsurprisingly revealing himself as a supporter of the old system, which he has claimed he wants to clean up.

And yet when the delight subsided I began to ask myself if Heydon Prowse had delivered such a public service after all. I started to wonder whether his secret filming was really in the public interest. Was it possible he had abused an important trust that must exist between journalists and politicians in a democratic society? Might it be that his underhand techniques, if repeated on a wider scale, would paradoxically have the effect of making politicians less prepared to speak frankly to journalists, and even, in the long-run, less accountable?

Political journalists, whether or not they are in the Lobby, recognise that ministers and shadow ministers are likely to reveal more information if they can speak unattributably. We may wish this was not so, but it is. Nor are these conventions limited to politicians. A newspaper editor whom I might telephone in connection with this column will usually ruminate more freely if he, or she, is confident of not being quoted directly. This is human nature.

Every week outside the silly season, dozens of lunches take place involving leading politicians and journalists, whose newspapers naturally pay. Before the sparkling water has been ordered, a politician who does not know and trust the journalist in question is likely to seek confirmation that the proceedings are off the record. Occasionally an even more watertight assurance of anonymity may be sought. Journalists accept this convention because it enables them to obtain information which would otherwise remain private.

Without such an agreement, be it tacit or explicit, journalists would be pariah figures, with little or nothing to write about. No lawyer, businessman, doctor, police officer or civil servant – let alone politician – would ever break bread with a journalist if he or she thought that what was said would immediately appear attributably in a national newspaper or on television.

Along comes Heydon Prowse, not a journalist but a self-styled campaigner who posts clips on YouTube. In fact, Mr Duncan was rather generous to invite him to the Commons, since he had dug a pound sign on the politician’s lawn, which became an internet hit. Mr Duncan was, however, very foolish to unburden himself to a previously unfriendly stranger as he did. Mr Prowse is not a man to consider that most of us say things in private that are flippant or jokey – about our employers, colleagues, friends or wider society – which do not fully represent our true, considered views.

There are fine lines to be drawn, I know. Not long ago undercover reporters from The Sunday Times posed as lobbyists in meetings with four Labour peers, who offered to help in ways that seemed at odds with proper Parliamentary practice. The reporters were wired for sound. Despite the apparent similarities with the case I have discussed, I would argue that the Sunday Times journalists were engaged in a legitimate undercover investigation into possible corruption, whereas Mr Prowse had no such aim in mind, and was on a general fishing expedition during which he was abusing Mr Duncan’s reasonable expectations of privacy. If he had stumbled across prima facie evidence that the politician was involved in illegal activity, I would say that the end justified the means. He didn’t.

I realise some people may think that Heydon Prowse and people like him are a breath of fresh air. Whereas journalists and politicians often operate in the shadows, Mr Prowse shamelessly uses a secret camera to record things as they are. (By the way, how was he able to circumvent the extremely tight security in the Commons?). I do not say that there are no dangers in the sometimes murky symbiotic relationship between MPs and scribes; and I can even see there is something refreshing about Mr Prowse’s approach. My point is only that if “citizen journalism” involves abusing conventions of privacy and confidentiality, politicians will speak less candidly to fewer people – and that is not in the public interest.

Boosting sales figures is no longer as easy as ABC

The Audit Bureau of Circulation (ABC) is sometimes considered a toothless tiger which simply writes down the sales figures given to them by newspapers, including foreign and “bulk” (ie free to the reader) sales. Not so, if a recent ABC inspection is any guide.

Four newspapers – the Daily Mail, Daily Telegraph, Financial Times and London Evening Standard – have been required to revise their circulation figures for the period October 2008 to April 2009 because they overstated their bulk sales. The size of the revisions varies. In the case of the FT they amount to fewer than 10,000 a month. As a proportion of total sales, the Mail’s revisions are slightly less, the Telegraph’s appreciably more.

The Standard has made by far the biggest revisions relative to its circulation. In January 2009 it claimed an overall sale of 292,976. This has been reduced by more than 55,000 to 237,403. Some advertisers may argue that the paper charged them at rates that presupposed a significantly higher circulation than the real one.

Who is at fault? The Mail and the Telegraph have issued a joint statement effectively blaming a distributor called Dawson Media Direct, which has just ceased trading as a separate entity. They suggest there was a “technical hitch”, which has now been put right, in the company’s supply to airlines.

It is difficult to believe that any of these titles deliberately inflated their bulk sales. That said, foreign sales remain a contentious issue. Newspapers are not obliged to prove that every copy sent abroad has been bought, on the basis that such an audit would be well-nigh impossible.

Some advertisers dislike bulk sales. The Guardian, which never went in for them in a big way, has recently announced it is ditching them. For higher-selling papers such as the Mail and Telegraph they still make commercial sense.